Conventionally, an X-ray diagnostic apparatus includes an X-ray beam limiting unit which can adjust an X-ray irradiation field. The X-ray beam limiting unit includes a plurality of filters (to be referred to as additional filters hereinafter) having different thicknesses to adjust the radiation quality of X-rays. Such an additional filter (also called a radiation quality filter or beam spectrum filter) can reduce soft X-rays generated by an X-ray generation unit in an X-ray diagnostic apparatus. The apparatus executes switching of a plurality of additional filters based on the object thickness estimated from conditions for the generation of X-rays (to be referred to as X-ray conditions hereinafter) and the like or the SID (Source Image Distance).
When switching (to be referred to as an object thickness switching method hereinafter) additional filters based on an estimated object thickness, the following problem arises. If the object thickness is small, the apparatus uses an additional filter exhibiting a high elimination efficiency of soft X-rays (X-rays having relatively long wavelengths and low penetrating powers) (in general, a thick additional filter). In contrast, if the estimated object thickness is large, since X-ray conditions (e.g., the maximum output of an X-ray tube and a restriction on the entrance X-ray dose on an object) tend to reach their upper limits at the time of fluoroscopy or the like, the apparatus uses an additional filter exhibiting a high X-ray transmittance (an additional filter exhibiting a low elimination efficiency of soft X-rays: a thin additional filter in general). If the object thickness is large, the X-ray diagnostic apparatus uses a thin additional filter in spite of the fact that it has a capacity enough to generate X-rays having high penetrating power. This leads to a problem that the maximum thickness of a filter which can be used for imaging or fluoroscopy becomes smaller than that of a filter which can be used for imaging or fluoroscopy by using the above capacity to the full.
A method of avoiding the above problem is a method of switching additional filters in accordance with the magnitude of SID (to be referred to as an SID-dependent switching method hereinafter). This method allows to ensure the maximum thickness of a filter which can be used for imaging and fluoroscopy by making an X-ray diagnostic apparatus use the capacity of generating X-rays having high penetrating power. However, the SID-dependent switching method uses a thin additional filter regardless of the object thickness if the SID is long, and hence the exposure dose becomes higher than that in the object-thickness-dependent switching method. In addition, the object-thickness-dependent switching method and the SID-dependent switching method have the problem of a deterioration in image quality.